Lubricating oils used in the internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. Among other adverse effects, this environment can lead to oxidative degradation of the oil. This oxidation of the oil may be catalyzed by the presence of certain impurities in the oil, such as iron compounds. This oxidation also may be promoted by the elevated temperatures to which the oil is subjected during use. The oxidation of lubrication oils during use is usually controlled in part by the use of antioxidant additives, which may extend the useful life of the oil, for example by reducing or inhibiting unacceptable increases in the viscosity of the oil.
Automatic transmission fluids should be oxidatively stable to maintain their frictional properties as the fluids are aged. To test the oxidative stability of these fluids, automobile manufacturers, such as General Motors, require that fluids be tested in oxidation tests and cycling tests. In these tests, the total acid number (TAN) of the oil is measured throughout the test, and at the end of the test the TAN of the oil must be within specified limits.
Existing lubricants employing diarylamine and a sulfurized compound are taught in U.S. Pat. Nos. 5,840,672, 6,174,842, and 6,326,336.
U.S. Pat. No. 5,073,278 teaches a lubricant composition containing an aromatic amine and a sterically hindered amine. The aromatic amine can be a ring-substituted alkylphenothiazine or nitrogen substituted alkylated phenothiazine.
U.S. Pat. No. 6,645,921 discloses a process for producing organomolybdenum compositions that are highly useful as lubricant additives. The described process involves reacting a fatty oil with a diamine, followed by reaction with a molybdenum source.
U.S. Pat. No. 6,599,865 discloses a combination of (1) an alkylated diphenylamine, (2) a sulfurized olefin/fatty oil and/or an ashless dialkyldithiocarbamate, and (3) an alkylated phenothiazine, which is highly effective at controlling crankcase lubricant oxidation and deposit formation.
What is needed is a fluid composition that remains oxidatively stable and maintains its frictional properties over time. Moreover, the fluid should meet the limits specified by the automobile manufacturers in terms of the total acid number (TAN) during and at the end of the test.